<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        input {
            border: 1px solid black;
        }
    </style>
</head>
<body>
    用户名：<input type="text" id="username">
    <p></p>
    <script>
        const input = document.querySelector("input");
        const p = document.querySelector("p");
        function handle(data){
           input.style.border = "1px solid red";
           p.innerHTML = data.msg;
        }
        input.onblur = function(){
            let username = this.value;
            const script = document.createElement('script');
            script.src = 'http://127.0.0.1:8000/check-username';
            document.body.appendChild(script);
        }
    </script>
</body>
</html>